1. Field of the Invention
The invention relates in general to a transmission method of a wireless signal and a transmitter using the same, and more particularly to a transmission method of a wireless signal which possesses time frequency diversity and facilitates channel estimation and a transmitter using the same.
2. Description of the Related Art
In a wireless communication system, a transmitter is utilized to transmit a wireless signal to a receiver in the form of an electromagnetic wave through physical channels such as air. Due to practical channel effects such as multipath reflection or propagation fading, the wireless signal received by the receiver may be distorted. If the multipath signal received by the receiver has a large delay spread, the coherent bandwidth of the multipath signal will be smaller than the coherent bandwidth of a single path signal, and the channel response of the multipath signal will result in frequency selective fading. Orthogonal frequency division multiplexing (OFDM) modulation technology based on multicarrier modulation is capable of resolving the problem of channel response of frequency selective fading and has thus become a mainstream technology in the application and development of wireless communication.
The OFDM modulation technology is used in wireless communication systems and digital audio and video broadcasting systems to perform high spectral efficiency transmission. The network framework of an OFDM system may be a multiple frequency network (MFN) or a single-frequency network (SFN). SFN is a broadcasting network, and all transmitters transmit the same signal through the same frequency channel at the same time. SFN has several advantages such as wide network coverage, excellent efficiency of frequency utilization, and the mobile user can receive the signal without switching to another frequency band in the network coverage as moving. Examples of OFDM-based SFN systems include digital video broadcasting-terrestrial (DVB-T), digital video broadcasting-handheld (DVB-H), digital audio broadcasting (DAB), digital multimedia broadcast-terrestrial (DMB-T) and media forward link only (Media-FLO).
OFDM in conduction with channel coding and time interleaving may enhance system performance. Even if error occurs because of part of the received signal with poor channel response, the erroneous bytes of the received signal still have chance to be corrected through channel decoding technology with reference to the other part of the received signal with better channel response. The functions of channel coding and time interleaving can further combine the diversity technology, such that the channel response of the received signal possesses diversity. Diversity transmitting/receiving is normally used in the OFDM system to provide larger channel diversity and enhance system performance with excellent diversity gain.
SFN used in the OFDM system has wide network coverage and many transmitters. At the cell edge between transmitters in the SFN, it may happen that a receiver receives the same signal from two transmitters almost simultaneously. The tiny delay spread results in flat fading channel response with a wide coherent bandwidth. If signals from the two transmitters have phase reversed to one another, their destructive combination hence results in a totally faded flat channel. It is even worse that, for a static/quasi-static receiver, this terrible situation may continue for a long time relative to the time interleaving length. Under such circumstances (flat and/or slow fading), a poor performance due to burst errors is expected for OFDM systems.
Referring to FIG. 1A and FIG. 1B. FIG. 1A shows a partial perspective of a conventional wireless communication system. FIG. 1B shows an example of flat fading channel response of the conventional wireless communication system. In the wireless communication system 100, the receiver 110 is located in the coverage boundary between the transmitter 102 of region A and the transmitter 108 of region B. The transmitter 102 and the transmitter 108 respectively transmit identical wireless signals 112 and 118 containing multiple pilot symbols p and multiple data symbols d (0), d (1), . . . , d (k). The channel response of the wireless signal 112 passing through region A is ha, and the channel response of the wireless signal 118 passing through region B is hb. If both the wireless signal 112 transmitted by the transmitter 102 and the wireless signal 118 transmitted by the transmitter 108 are s00, then the wireless signal 120 received by the receiver 110 is s01, which is expressed as:s01=s00×(ha+hb)
As the wireless signals 112 and 118 correspond to a smaller multipath delay spread and have reversed phase rotation, destructive interference may occur. When the channel response ha is approximately equal to −hb, the receiver 110 will generate a flat fading channel response (ha+hb˜0) with low amplitude. Consequently, the coherent bandwidth is huge and channel response is lacking of diversity. Furthermore, the value of the low channel response 122 of the received signal caused by destructive interference may be smaller than the threshold value 124 of the signal detector of the receiver 110, hence resulting that the received signal cannot be correctly detected, and the receiving function of the wireless communication system is largely degraded. Therefore, it is important to ‘create’ diversity for solving the problem without affecting the receiver design (i.e., backwards compatible).
Diversity technology avoids the occurrence of low and flat channel response. Referring to FIG. 2, an example of channel response of the conventional wireless communication system adopting group scrambling diversity technology is shown. In FIG. 2, a group scrambling method is adopted to divide multiple subcarriers into groups. For example, the signal transmitted by the first transmitter is divided into subcarrier groups 211˜212, and the signal transmitted by the second transmitter is divided into subcarrier group 221˜222, wherein each subcarrier group includes multiple data symbols and multiple pilot symbols.
Each subcarrier group respectively encodes multiple data symbols and multiple pilot symbols with different scrambling symbols. Thus, in the coverage boundary between two transmitters, the receiver experiences a channel response 240 with diversity when receiving a wireless signal 230. For the channel response 240, each subcarrier group is independent to one another, and thus frequency diversity gain is obtained. However, the combined channel response 240 has discontinuity, so the group scrambling method is unfavorable to the channel estimation in the frequency domain, hence decreasing the accuracy and increasing the complexity of the channel estimation of the receiver. Besides, the group scrambling method cannot obtain time diversity gain against slow fading.
Referring to FIG. 3, another example of channel response of the conventional wireless communication system adopting grid scrambling diversity technology is shown. In FIG. 3, a grid group scrambling method is adopted to divide multiple subcarriers into grids in time and frequency dimension. For example, the signal transmitted by the first transmitter during the time period C is divided into grid 311˜312, the signal transmitted by the first transmitter during the time period D is divided into grid 313˜314, the signal transmitted by the second transmitter during the time period C is divided into grid 321˜322, and the signal transmitted by the second transmitter during the time period DI is divided into grid 323˜324, wherein each grid includes multiple data symbols and multiple pilot symbols.
Each grid respectively encodes multiple data symbols and multiple pilot symbols with different scrambling symbols. Thus, in the coverage boundary between two transmitters, the receiver experiences a channel response 340 with diversity when receiving a wireless signal 330. For the channel response 340, each grid is independent to one another, and thus frequency diversity gain is obtained and destructive signal interference is avoided. Moreover, the grid group scrambling method obtains time diversity gain against slow fading nature. However, the combined channel response 340 has discontinuity, so the grid group scrambling method is unfavorable to the channel estimation, hence decreasing the accuracy and increasing the complexity of the channel estimation of the receiver.